The present invention relates to an apparatus for perceiving an intruder into an area by detecting infrared rays being radiated from the intruder.
All the bodies at an ordinary temperature, whether animate or inanimate, radiate infrared rays having a peak spectrum around 10 .mu.m. This phenomenon is applied to various kinds of systems for perceiving a person intruding into a specific area or passing through a predetermined position. A typical example of such intruder perceiving system is described in accordance with FIGS. 6 and 7. In FIG. 6, which schematically shows the aspect of a conventional system for perceiving a person P intruding into a predetermined domain D on the ground (or floor) G, there are supposed within the domain D a plurality of discrete sub-domains, for example, three separate sub-domains D1, D2 and D3, and an infrared detecting apparatus 1 is located at a suitable height h above the ground G. The infrared detecting apparatus 1 is devised so as to detect the total of only the (infrared) rays falling thereto within detectable-rays enveloping slender spaces S1, S2 and S3 which are defined by the geometrical envelopes spanned between the infrared receiving portion 11 of the infrared detecting apparatus 1 and the circumferences of the sub-domains D1, D2 and D3. In case there are no intruders in the domain D, the infrared detecting apparatus 1 outputs as a background signal the total signal corresponding the infrared rays radiated form the sub-domains D1, D2 and D3. When a person P having a height h.sub.p intrudes into the domain D and crosses the detectable-rays enveloping spaces S1, S2 and S3 one after another, the output from the apparatus 1 varies as indicated by a solid line in the graphic representation shown in FIG. 7. Such an output from the infrared detecting apparatus 1 can be used, for instance, to trigger an alarm signal generating device (not shown), thereby enabling to constitute a burglar alarm system. In this case the alarm signal generating device must, of course, be devised so as to be triggered by a signal level not higher than the (lowest) level indicated with a one-dot chain line shown in FIG. 7. However, the intruder perceiving system shown in FIG. 6 has an important disadvantage that the alarm is undesirably put into action against the intrusion of a small animal short of stature such as a dog or the like because the infrared detecting apparatus 1 responds also to such a small animal and outputs signals with a maximum level exceeding, as shown with dotted lines in FIG. 7, the above-mentioned alarm system triggering level. Such a disadvantage may be cleared, for example, by increasing the area of each of the sub-domains D1, D2 and D3, as is illustrated in FIG. 8. In the figure a representative enlarged sub-domain (denoted by Ds representing D1, D2, D3 of FIG. 1) has an area much larger that that occupied by an animal A, and symbol S.sub.d, S.sub.p and S.sub.a respectively stand for the cross-sectional area of the detectable-rays enveloping space Sr (representing S1, S2, S3 of FIG. 6) and those of the infrared radiation fluxes from a person P and the animal A. As is understood from FIG. 8, an increased area of the sub-domain Ds comes to make a relation S.sub.a /S.sub.d &lt;S.sub.p /S.sub.d exist. This means that the relative intensity of the infrared radiation from the animal A is favorably lower than that from the person P. However, the increased area of the sub-domain unfavorably decreases also the relative infrared radiation from the person. According to the above favorable and unfavorable effect, the graphic representation of the output from the infrared detecting apparatus 1 can be illustrated as shown in FIG. 9, which corresponds to FIG. 7. Though the higher output level in case of a person (solid line) is distinguished from the lower output level in case of a small animal, as is obvious from FIG. 9, the clearance d between the minimums of the former and the maximums of the latter is made small. This makes it critical to determine a signal level to steadily triger a following apparatus such as a burglar alarm. In addition a decreased level of the output (in the case of the intruder being a person) also is very disadvantageous.
Moreover, in accordance with the above-mentioned intruder perceiving system as shown in FIG. 6, the conventional infrared detecting apparatus 1 employed therein must comprise, whether the areas of the sub-domains are increased or not, a complex lens assemby consisting of many infrared converging lenses which have their respective principal axes directed to their corresponding sub-domains supposed on the ground G. The complex lens assembly consisting of many lenses makes the mechanical constitution of the apparatus 1 very complicated.